The present invention relates to a steel melting and secondary-refining method in which the steel is melted in an electric furnace and the thus obtained molten steel is refined in ladles.
The following methods have been conventionally utilized for refining molten steel.
.circle.1 A stream degrassing process (cited from "Progress of Steel Vacuum-degassing Method", THE IRON AND STEEL INSTITUTE OF JAPAN) (see FIG. 1); After melting, oxidation, decarburization, and deoxidation have been performed in an electric furnace, stream vacuum-degassing is performed mainly in a process of transferring molten steel from a ladle to another one. The method has no special refining function other than degassing.
In this method, the electric furnace has poor productivity, high running cost of electric power or the like, and low refining ability.
.circle.2 An ASEA-SKF method (cited from ASEA Journal, No. 6-7, 39) (see FIG. 2): After melting, oxidation, decarburization, temperature rising, and pre-deoxidation have been performed in an electric furnace, vacuum degassing, induction stirring, and reheating by electric arcs are performed in a refining ladle.
Since a pre-deoxidation process is performed in the electric furnace, high productivity is not obtained by the electric furnace. Further, since dephosphorization is performed by repetition of slag-making and slapping-off processes in the electric furnace, the dephosphorization affects the refining level, refining cost, and electric furnace productivity. In secondary refining, the efficiency of temperature rising is remarkably poor because reheating is performed by arcs, and the productivity is low. Further, the cost of elecric power as well as the cost of subsidiary materials (electrodes and refractories) are high.
.circle.3 An LF (Ladle Furnace) method (cited from "Iron and Steel Making Method", THE IRON AND STEEL INSTITUTE OF JAPAN) (see FIG. 3): After melting, oxidation, decarburization, temperature rising, pre-deoxidation have been performed in an electric furnace, reduction refining and reheating are performed in a refining ladle.
This method is equivalent to the ASEA-SKF method from which the vacuum equipment is removed. Therefore, similarly to the ASEA-SKF method, the method has low productivity and high cost of electric power as well as in cost of subsidiary materials. Moreover, in the method, no degassing function exists, and dephosphorizing ability is very low.
.circle.4 A vacuum-degassing and bubbling method under existence of slag (cited from Japanese Patent Unexamined Publication Nos. 192214/82 and 73817/86) (see FIG. 4): After melting, oxidation, decarburization, temperature rising, and pre-deoxidation have been performed in an electric furnace, reducing slag is added into a refining ladle, and stirring and vacuum treatment are simultaneously performed under an inert gas such as an Ar gas or the like.
The effects of deoxidation, inclusion-removal, degassing, and the like are remarkable and the reaction speed is very high, so that reheating is not required and it is possible to apply this method to continuous casting. However, the productivity of an electric furnace is not high, similar to the other methods. Further, dephosphorizing ability is not satisfactory since molten steel is tapped after oxidation and deoxidation at a high temperature.
The problems in the conventional melting and refining techniques as described above are summarized as follows.
.circle.1 The ability of melting equipment (mainly, an electric furnace) is not exhibited at its maximum. This is because in the conventional technique treatments such as oxidation, decarburization, temperature rising, slagging-off, pre-deoxidation, and the like are performed after melt-down; and thereafter molten steel is tapped.
.circle.2 The dephosphorizing ability is low. Therefore, it is necessary to perform slag-making and slagging-off process once or more after melt-down. That is, in the conventional technique, molten steel must be tapped at high temperature because the temperature falling is remarkable in secondary refining after the molten-steel is tapped. This is because as the temperature of molten steel rises, the equilibrium distribution coefficient of phosphorus (LP) to slag and molten steel is reduced along the curve in FIG. 5 in accordance with the equation (1) in the same figure.
.circle.3 The refining cost in the electric furnace step and the secondary refining step after melt-down is extremely high. In the conventional technique, (a) the temperature rising by electric arcs in the electric furnace and secondary refining furnace is low in energy efficiency (about 25%). Therefore, the treatment time is long and the consumption of electrode rods, refractories and the like is large, so that the refining cost is high. Further, (b) the process of oxidation and decarburization.fwdarw.dephosphorization (slag making.fwdarw.slapping off).fwdarw.temperature rising.fwdarw.pre-deoxidation.fwdarw.tapping of molten-steel.fwdarw.secondary refining (slag making .fwdarw.deoxidation.fwdarw.desulfurization.fwdarw.degassing.fwdarw.removin g inclusion.fwdarw.temperature rising) is progressed stepwise and serially with respect to the whole quantity of molten steel. Therefore, it takes a long time from melting down to the end of refining, and the various costs becomes relatively high.